The goal of the proposed research is to describe the epidemiology of cigarette smoking and its association with tuberculosis (TB) among injection drug users (IDUs) while accounting for the effect of HIV infection and patterns of illicit drug use Cigarette smoking is associated with multiple TB outcomes, including active TB disease, relapse, and TB mortality. However, despite the high prevalence of cigarette smoking among IDUs, it is unknown whether cigarette smoking is associated with increased TB infection among IDUs, many of whom are already at high risk of TB due to HIV infection and the use of illicit drugs. Furthermore, little is known regarding the patterns of cigarette smoking behavior and readiness to quit smoking among IDUs. In October 2010, Mexico enacted a new drug reform law that will significantly expand methadone maintenance treatment for opiate-addicted persons, which presents a timely opportunity to prevent TB among IDUs in drug treatment programs. The proposed research seeks to inform the integration of tobacco dependence treatment into drug treatment programs by accomplishing the following aims: 1) determine whether cigarette smoking is associated with TB infection among IDUs in Tijuana, Mexico, independent of HIV status and patterns of illicit drug use, including injecting illicit drugs and smoking marijuana, crack cocaine, and methamphetamines; 2) describe the relationship between the dosage and duration of cigarette smoking and TB infection and to evaluate whether a dose-response relationship exists; and 3) describe cigarette smoking behavior and readiness to quit among IDUs who currently smoke cigarettes. We will accomplish these aims by using the baseline data of a large, ongoing cohort study of HIV risk and drug use among IDUs in Tijuana, Mexico (PI: Strathdee, NIH R37 DA019829). The parent study includes data on HIV status and extensive questions on illicit drug use patterns, which will allow us to examine the association between cigarette smoking and TB infection while adjusting for HIV status and the use of illicit drugs (Aims 1 and 2). We will also address an important limitation of previous studies of cigarette smoking and TB infection by employing biological measures for cigarette smoking (a semi- quantitative urinary cotinine assay) and TB infection (an interferon gamma release assay) for a subset of 350 participants. For Aim 3, we will take advantage of the full baseline dataset of the parent study (N=815) to be among the first to describe tobacco dependence and readiness to quit among IDUs. The PI will conduct the proposed research as part of his doctoral dissertation under the supervision of his dissertation committee and other mentors. Conducting this research will help the PI gain expertise in the interaction between drug and tobacco dependence and infectious diseases such as HIV and TB, and will provide a strong foundation for a successful career as an independent researcher. Furthermore, the proposed research will provide essential data to inform the development of tobacco dependence treatment programs for IDUs in Mexico and elsewhere.